Vectoring technology cancels the FEXT (far-end crosstalk) between DSL lines, and therefore maximizes DSL system performance. Vectoring technology enables offering 100 Mbps per user with DSL lines e.g. between the end of a fiber network and the Customer Premises Equipment, CPE.
The Telecommunication Standardization Sector of the International Telecommunication Union, ITU-T, has been standardizing a vectoring standard G.993.5 [1], and the first recommendation of G.993.5 was approved on Apr. 22, 2010. The cancellation of the FEXT is done at the DSLAM (Digital Subscriber Line Access Multiplexer) side. The downstream FEXT is pre-cancelled by a precoder in the DSLAM, while the upstream FEXT is cancelled by an upstream crosstalk canceller in the DSLAM. The recommendation provides a way to estimate the FEXT channel in both downstream and upstream and utilize the estimated channel to cancel the crosstalk.
A Disorderly Leaving Event, DLE, which may alternatively be denoted e.g. Disorderly Shutdown Event, DSE, on a DSL line occurs e.g. when a user unplugs the telephone cable or turns off the CPE abruptly. The disorderly shutdown of a DSL line may change the crosstalk channel characteristics, i.e. the crosstalk coupling to other lines, due to the impedance change at the CPE end which is disorderly shut down.
However, when using vectoring, the precoder in the DSLAM remains unchanged after a DLE and continues to be optimized for the original channel characteristics, i.e. the channel characteristics before the DLE. This could result in a significant SNR (Signal to Noise Ratio) drop for other lines, since the precoder is outdated and thus cannot completely cancel the crosstalk from the line which is disorderly shut down. A DLE on one line can make other lines retrain. In VDSL2, retraining a line may take 30 seconds, which is a considerable interruption, e.g. in IP-TV services. Even though the retrain time is significantly shorter in G.fast, it is still several seconds, which would still cause undesirable service interruption.
A method for handling DLEs is presented in [1], which is the work of two of the inventors of the present disclosure, and which is incorporated herein by reference. According to this method, a partial channel estimate is derived after the DLE, and is combined with a channel estimate derived before the DLE. That is, a part of the original channel estimate, in form of a channel coefficient matrix, is replaced with a new estimate, e.g. a column of the channel coefficient matrix. This method works very well for frequencies where the crosstalk between lines is within certain limits. However, when using higher frequencies for communication, such as in G.fast, the crosstalk between lines is larger, and thus all parts of the channel coefficient matrix are affected to a larger extent, even though the change in some parts may still be dominant. Measurements indicate that the impact of DLE is serious for frequencies around 30 MHz and beyond, which is of interest for G.fast.
Thus, there is a need for a fast method for estimating a channel, a channel tracking method, which works well also for higher frequencies.